1. Field of the Invention
This invention relates to a shield tunneling machine for excavating a tunnel and, more particularly, to a shield tunneling machine for excavating a tunnel while excavating the tunnel face with a cutter head provided with two types of bits.
2. Description of the Prior Art
Generally, a shield tunneling machine for excavating a tunnel by use of the pressurized muddy water or clear water while preventing collapse of the tunnel face mounts on a cutter head either a plurality of cutter bits used for excavating a soft layer like clay layer or a plurality of roller bits used for excavating a hard layer like bedrock layer.
However, when the machine provided with said cutter bits excavates the hard layer, the cutter bits are damaged by the tunnel face and when the machine provided with said roller bits excavates the soft layer the efficiency of operation is degraded. Thus, the general machine provided with only one type of bits can excavate only one of soft and hard layers according to the type of bits mounted on the cutter head.
Some excavating machines having the cutter head provided with a plurality of cutter bits and a plurality of roller bits are known. However, in these well-known machines the respective cutter bits and roller bits are fixed to the cutter head so that clay, mud, etc. are attached to the roller bits in excavating the soft layer and thus disadvantageously the roller bits hinder the excavating operation and the cutter bits are damaged in excavating the hard layer.